You can use your imagination to its fullest in SL, either have a moonbeam touch your girlfriend on a starry night and her magically become pregnant, or you can roleplay finding a baby under a gooseberry bush.

Or you can go to the SL marketplace, or inworld searches and look up "baby" or "pregnant", or similar terms and see what is on offer to aid your way in getting your GF pregnant.

Good luck to you, and may you have a happy lifetime of bringing up many children together.